Would it make sense to create UOM for "bundle" (or use an existing one), then maintain the conversion rule 1 bundle = 25 PC in the material master and make bundle the sales UOM? Since you are, in fact, selling bundles and not PC.
If down the line these bundles still need to be assembled (although in this case I'm not sure why you are selling goods like that if they are packaged differently), then perhaps bundles can be converted back to PC. On any paperwork you can easily show PC using UOM conversion.
It'd make sense to think what exactly you're trying to achieve at the end, not just "how to restrict order". It's just a small part.